Wat's Dyke is a 40 mile (64 km) earthwork running through the northern Welsh Marches from Basingwerk Abbey on the River Dee estuary, passing to the east of Oswestry and onto Maesbury in Shropshire, England.
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Coastal Wales along the Dee Estuary was still in Mercian hands in 821, as it is known that Coenwulf died peacefully at Basingwerk in that year.
Coastal Wales along the Dee Estuary must have remained under Mercia through 821, as Coenwulf is recorded dying peacefully at Basingwerk in that year.